The Oxford Thesaurus An A-Z Dictionary of Synonyms - serious adj.

1) grave, solemn, earnest, unsmiling, poker-faced, straight-faced, sedate, sober, pensive, thoughtful; humourless, sombre, grim, dour, severe: One should be wary of the person who is always serious, who cannot see the ludicrous side of life. 2) grave, important, vital, dangerous, weighty, significant, momentous, crucial, consequential, life-and-death or life-or-death, urgent, pressing; no laughing matter, of consequence or moment or importance: Things like the greenhouse effect and the hole in the ozone layer are serious concerns for all mankind. 3) sincere, straightforward, not joking or fooling, genuine, honest: Is he serious about wanting me to fetch him a left-handed wrench? 4 acute, critical, life-threatening, bad, dangerous, nasty, perilous, alarming, grave, severe, precarious: His condition is serious and we might have to operate tonight. She had no idea that measles could be such a serious illness.